Asp.net - Creating Multiple Querystrings From Drop Down Lists - Could Be 1, 2 Or 3 Querystrings?

I have a gridview which can be filtered from one or more values in a querystring. That all works great: e.g. "?subject=Maths&authorName=Bond_James&type=Magazine" The values passed to the query string come from 3 drop down lists: Subject, Author, Type.What I'd like is when the user presses "Filter" it will take the selected values from the drop down lists and pass them to the querystring - it could be 1 value, 2, or all 3 (like above).

The drop down lists have an item called "All Subjects" / "All Author" / "All Type" each with a value of -1. The idea being that if the user leaves these items selected then the Filter button just ignores them.

Here is my code so far:

Protected Sub buttonFilterGo_Click(ByVal sender As Object, ByVal e As EventArgs) Handles buttonFilterGo.Click
Dim queryString As String
Dim selectedAuthorString As String = "author=" & dropAuthorList.SelectedItem.Value

[code]....

Also, one more thing. How do I get the drop down lists to have the filters selected when the page re loads?

EDIT: I changed the default values of the drop down lists to "" - this leaves the URL looking messy though ?author=&subject=&type= This works, is it the best way?